The basic task of electrical transformers is to allow for the exchange of electric energy between two or more electrical systems of usually different voltages. In practice, a transformer converts electricity at one voltage to electricity at another voltage, either of higher or lower value.
Most common electrical transformers generally include a magnetic core composed by one or more legs or limbs connected by yokes which together form one or more core windows. Around the legs there are arranged corresponding primary and secondary coil assemblies, wherein each coil assembly is composed by one or more phase windings, for example, low-voltage windings, and/or high-voltage windings. The phase windings are usually realized by winding around a mandrel suitable conductors, for example foils, wires, or cables, or strips, so as to achieve the desired number of turns.
Some known winding techniques used to form coils are the so-called foil winding and disc or foil-disc winding techniques. In practice, in the foil winding technique, a full-width foil of electrical conductor is used, while in the disc or foil-disc winding technique, a portion of the foil is used, namely having a width corresponding to that of the disc to be wound.
The type of winding technique that is utilized to form a coil is primarily determined by the number of turns in the coil and the current in the coil.
For high voltage windings with a large number of required turns, the disc or foil-disc winding technique is generally used, whereas for low voltage windings with a smaller number of required turns, the foil winding technique is generally used.
One important aspect in manufacturing electrical transformers resides in its capability to be cooled. During operation, electrical transformers generate a substantial amount of heat which should be dissipated as much as possible in order to avoid overheating that would negatively affect the electrical performances of the transformers.
In order to achieve the needed cooling, a known solution consists in including into the windings one or more cooling sectors or ducts defined between adjacent turns. A cooling fluid, such as air in the case of dry-type transformers, circulates inside these cooling sectors or ducts.
The embodiment of cooling sectors or air ducts into the windings is to some extent rather difficult and cumbersome, especially when turns are wound in a disc-type configuration.
Further, the inclusion of air ducts in a winding of a dry-type transformer can result in a difference in electrical capacitance between the two adjacent turns delimiting the cooling sector or air ducts and the rest of the turns themselves. This results in an uneven voltage distribution over the turns during high frequency voltage surges, for example, lightning impulses, and can lead to breaks of the insulating material in the cooling sector of air ducts.